Method of injection molding utilizing pressurized fluid source within a chamber in a mold

ABSTRACT

A method and system for the injection molding of plastic articles in an injection molding system including a chamber in a mold of the system which stores a charge of pressurized gas immediately prior to use. The mold has an injection aperture and a mold cavity for receiving molten resin. A resin flow path fluidly communicates the injection aperture and the mold cavity within the mold. An orifice in the mold opens the chamber to the mold. A gas receiver initially stores the charge of pressurized gas and a fluid circuit selectively introduces the charge of pressurized fluid into the chamber. A valve mechanism in the chamber selectively fluidly communicates the chamber to the flow path under control of a controller. When communicated to the flow path, the charge of gas distributes the molten plastic within the mold cavity in one embodiment of the invention.

Cross-Reference To Related Applications

This application is a C-I-P application of U.S. Ser. No. 674,997 filedMar. 28, 1991 entitled "Method and System for the Injection Molding ofPlastic Articles Utilizing a Fluid Compression Unit", which, in turn, isa C-I-P application of U.S. Ser. No. 552,909, filed Jul. 16, 1990entitled "Method and System for the injection Molding of PlasticArticles Utilizing a Fluid Compression Unit", both of which areincorporated herein by reference.

TECHNICAL FIELD

This application relates to method and systems for injection molding ofplastic articles using fluid pressure and, in particular, to method andsystems for the injection molding of plastic articles using fluidpressure to assist in the making of plastic articles.

BACKGROUND ART

It is known in the plastic molding art to use pressurized fluid inconjunction with the plastic molding of articles, as disclosed in theU.S. Pat. to Friederich 4,101,617.

Gas-assisted injection molding is a thermoplastic molding process whichprovides stress-free large with a class A surface and virtually no sinkmarks. Gas-assisted injection molding is a low-pressure molding processcompared to conventional injection molding. In this process, inert gasis injected into the plastic after it enters the mold. The gas does notmix with the plastic but remains in the middle of the thicker sectionsof the molding. By controlling the gas pressure, the quantity of plasticinjected into the mold (short shot) and the rate of gas flow, apredetermined network of hollow interconnecting channels is formedwithin the molded part. The gas pressure remains constant in the networkof hollow channels throughout the molding. This compensates for thetendency of the plastic to shrink at the thicker areas of the molding,preventing warpage and reducing stress The gas pressure is relieved justprior to opening the mold. Because of the relatively low injectionpressure, large parts can be molded with substantial reductions in clamptonnage.

The gas system equipment provides the precise control of pressure,timing and volume of gas which is injected into the part, all of whichare important to the control of the gas-assisted injection process

In U.S. Pat. No. 4,948,547 entitled "Improved Method for the Use of GasAssistance in the Molding of Thermoplastic Articles," assigned to theAssignee of the present invention, a method of gasassisted injectionmolding is disclosed in which a charge of pressurized gas is injectedinto the mold but not into the article-defining cavity. The gas chargeis of a predetermined quantity and pressure, sufficient to assist infilling out the article defining cavity with resin and promoting surfacequality.

U.S. Pat. No. 4,935,191 discloses a process for producing an injectionmolded product including introducing a stream of plastic material into amold space at a first pressure. A quantity of gas is stored in a storagechamber at a second pressure which is at least as high as the firstpressure. The gas is introduced into the molten stream of plasticmaterial immediately after the molten material has passed the positionat which the gas is introduced, thereby forming a gas cavity in themolten material.

The mechanism for charging a pressurized fluid or gas for use in amolding process is described in detail in U.S. Pat. No. 4,855,094. Also,a detailed description of various mechanisms for introducing thepressurized gas into the mold is described in this patent which isassigned to the Assignee of the present application. One mechanism is asliding needle valve located in a sprue bushing. In an open position ofthe needle valve, gas is injected through a port.

One limitation of the prior art is that a charge of pressurized gas isstored some distance from the mold and, consequently, there are pressuredrops as the gas travels to the mold. Repressurization must then occur.Another limitation of the prior art is that unless some mechanicalmechanism is provided, the pressurized gas must have a pressure lessthan the pressure of the molten plastic in the mold.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and system forthe injection molding of plastic articles wherein a charge ofpressurized fluid can be stored in the mold and have a pressure higherthan the pressure of the molten plastic in the mold.

Another object of the present invention is to provide a method andsystem for the injection molding of plastic articles wherein a movablevalve mechanism forms a depression in the molten plastic in the moldwhich helps to direct the flow of pressurized fluid allowed to flow uponmovement of the valve mechanism.

Yet still another object of the present invention is to provide a methodand system for the injection molding of plastic articles wherein acharge of pressurized fluid is available immediately adjacent the moltenplastic in the mold during the injection molding process.

In carrying out the above objects and other objects of the presentinvention, a method for the injection molding of plastic articles in aninjection molding system is provided. The injection molding systemincludes a mold having an injection aperture and a mold cavity forreceiving molten resin. The method includes the step of injecting aquantity of molten plastic through the injection aperture and into themold cavity. A resin flow path fluidly communicates the injectionaperture and the mold cavity within the mold. The method also includesthe step of communicating a charge of pressurized fluid from a source ofpressurized fluid to a chamber within the mold, an orifice opening fromthe chamber to the flow path, and blocking the orifice so that thecharge of pressurized fluid is stored within the chamber. The methodfurther includes the steps of unblocking the orifice so that the storedcharge of pressurized fluid is introduced into the flow path from theorifice, containing the fluid under pressure within the mold until thearticle is set up and formed in the mold cavity, and venting the fluidfrom the mold.

Further in carrying out the above objects and other objects of thepresent invention, an injection molding system for the injection moldingof plastic articles is provided. The system includes a mold having aresin injection aperture and a mold cavity for receiving molten plastic.The system further includes a resin flow path fluidly communicating theinjection aperture and the mold cavity with the mold, a chamber and anorifice opening from the chamber to the flow path. Fluid receiver meansstores a charge of pressurized fluid. Fluid circuit means interconnectsthe fluid receiver means to the chamber for selectively introducing thecharge of pressurized fluid into the chamber. Valve means selectivelyfluidly communicates the chamber to the flow path. Control meansconnected to the valve means and controlling operation thereof causesthe valve means to initially store the charge of pressurized fluid inthe chamber and then causes the valve means to communicate the chamberto the flow path to allow the charge into the flow path.

Preferably, the mold includes valve means having a piston and a tipportion operatively associated the piston to move therewith within thechamber. The tip portion has a closed position and an open position tointroduce the charge of pressurized fluid from the chamber, through theorifice and into the flow path. The stored charge applies a pressure tothe piston to urge the tip portion toward its open position.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic view of a system constructed in accordancewith the present invention;

FIG. 2 is an enlarged sectional view of a sprue bushing having a valvemechanism disposed therein in a closed position;

FIG. 3 is a view, partially broken away and in cross-section, of thesprue bushing with molten plastic in the sprue and the valve mechanismin an open position;

FIG. 4 is a view similar to FIG. 3 illustrating the prior art;

FIG. 5 is a view, partially broken away and in cross-section, whereinthe sprue bushing of FIGS. 1-3 has been modified for placement adjacentthe mold cavity or a runner system of the mold;

FIG. 6 is a view of the modified sprue bushing of FIG. 5 with its valvemechanism in an open position to allow the charge of pressurized gas toflow into a cavity in the mold; and

FIG. 7 is an end view, partially broken away, of the modified spruebushing of FIGS. 5 and 6.

BEST MODE FOR CARRYING OUT THE INVENTION General Method Of Operation

In overview, the method provides for molding of plastic articles withhollow interior sections where pressurized fluid is present in formationof the article in the mold cavity. The presence of the pressurized fluidcreates an outward pressure which minimizes sink marks and reduces thematerial requirements and weight of the molded article. However, it isto be understood that the method is capable of providing for the moldingof plastic articles with solid interior where pressurized fluid isinjected into the mold but not into the article-defining cavity. The gascharge is of predetermined quantity and pressure sufficient to assist infilling out the article-defining cavity with resin and promoting surfacequality as described in greater detail in U.S. Pat. No. 4,948,547,entitled "Method for the Use of Gas Assistance in the Molding of PlasticArticles".

As disclosed in the above-noted patent applications, a charge ofpressurized fluid is stored within a fluid receiver means or mechanism10 at a level within a predetermined range of pressures such as 14,000psi. This pressure may be reduced to the pressure required by theinjection molding process such as by a gas regulator 12.

In one embodiment, a quantity of molten plastic is injected from thenozzle of a conventional injection molding machine to a flow path 14 ina gas injection bushing, generally indicated at 16. The molten plasticflows along a resin flow path into a mold cavity of a mold at aninjection pressure. The quantity of molten plastic, i.e. the plasticshot, is less than the quantity of plastic which would ordinarily berequired to fill the mold cavity.

The charge of pressurized fluid, preferably nitrogen gas, iscommunicated from the fluid receiver means 10 during or before theplastic injection, to a gas chamber 18 in selective fluid communicationwith the mold cavity through an orifice 20 which open from the chamber18 to the resin flow path. The orifice does not have to have asufficiently small dimension to resist entry of the relatively viscousmolten plastic, as disclosed in U.S. Pat. No. 4,943,407, since theorifice is closed by the tip portion 22 of a valve means or mechanism,generally indicated at 24. The tip portion 22 extends into the spruearea or flow path 14 so that the charge of pressurized gas is introducedthrough the orifice 20 into the center of the sprue and then down thecenter of the sprue. This is to be contrasted with the prior art methodof FIG. 4 (i.e. as shown in U.S. Pat. No. 4,943,407, where gas pushesthe sprue sideways and then the gas finds its way into the molded sprue.

Then, the pressurized gas is contained under pressure within the mold,until the article has set up in the mold cavity. During the formation ofhollow plastic articles, the pressurized gas exerts outward pressurewhich forces the plastic to conform to the detail of the mold surfaceand exhibit fine detail with minimal sink marks or other surfacedefects. Preferably, a relatively low holding pressure is maintained inthe mold to prevent a turbulent venting of the gas to ambient.

Finally, the gas is vented from the mold to ambient or to a gas holdingtank for reuse prior to opening the mold and removing the finishedmolded article.

Description Of The System

As previously mentioned, pressurized gas is stored in the gas or airpressure receiver 10 which preferably has a 14,000 psi capacity.

The pressure of the gas from the pressure receiver 10 can be reduced orregulated to the proper pressure needed by the injection molding processby the pressure regulator 12. The regulator 12 reduces the 14,000 psi toa value needed for the article or molding i.e. 1,000 psi to 8,000 psiand, preferably, 2,500 to 3,500 psi depending on the plastic used forthe article.

A charge of pressurized gas needed for the injection molding process istransferred or communicated to the chamber 18 when the solenoid of adirectional control valve 26 is energized under control of a controller28 and is open long enough to communicate and direct a charge of gaswithin the pressure gas receiver 10 through a passage 30 to the chamber18. Because the tip portion 22 of the valve mechanism 24 blocks theorifice 20, the charge of pressurized gas is stored in the chamber 18 onone side of a piston 32 of the valve mechanism 24.

When a sufficient amount of gas has been communicated from the gasreceiver 10, the directional control valve 26 is deenergized.

Prior to the charge of gas being communicated to the chamber 18 on oneside of the piston 32, gas is communicated to the opposite side of thepiston 32 through a passage 34. This gas comes from the receiver 10after being reduced or regulated by a gas regulator 36. A directionalcontrol valve 38 is energized under control of the controller 28 and isopen long enough to communicate an amount of gas on the opposite side ofthe piston 32 to hold the tip portion 22 in its closed position to storethe charge of gas in the chamber 18.

When the stored charge of gas is to be introduced into the mold cavity,the solenoid of a directional control valve 40 is energized also undercontrol of the controller 28 and is open long enough to vent the gas onthe opposite side of the piston 32 through the passage 34 and toatmospheric pressure.

A gas directional valve 42 under control of the controller 28 allows gasin the article to flow to a pressure reducing valve or pressureregulator 44 to allow a low holding pressure in the molding i.e. justenough pressure to complete the molding and hold the thick sections fromsinking.

A manual throttle valve 46 controls the rate of gas coming out of themolding. This eliminates fast turbulent venting of the gas from themolding or article.

After the fluid has been contained under pressure to allow the articleto set up in the mold cavity, the fluid is vented to ambient or to areservoir or tank by energizing a directional control valve 48 to openthe valve 48. The gas from the mold may be controlled at a metered rateby a manual gas throttle valve (not shown) so that the gas pressuredoesn't go down to atmospheric pressure too fast. Thereafter, thedirectional control valve 48 is closed and the completed article isejected from the mold.

Alternatively, the fluid may be vented to ambient by separating the moldand the injection nozzle (not shown) after the molten resin has cooledbeneath its softening point.

Sequence of Operations

(1) Energize the valve 38, gas goes to the opposite side of the piston32 closing the valve mechanism into the position shown in FIGS. 1 and 2.

(2) Inject plastic into resin injection aperture and into mold cavity.

(3) Any time during or before or after π2 step, energize valve 26allowing gas at pressure setting of regulator 12 to enter the chamber18. The valve mechanism 24 will not move as the area difference betweenopposites sides of the piston 32 will keep the tip portion 22 of thevalve mechanism 24 closed even if the pressure of the regulator is afraction of the pressure of regulator 12. However, the pressure of theregulator 36 must be high enough to overcome the pressure of theregulator 12.

(4) Once in the chamber 18, the gas pressure to inject is closer to themolding then shown by the prior art and at a moment's notice ready toinject into the molding with little if any gas pressure loss. Typically,it takes only 0.02 cubic centimeters of gas to move the valve mechanism24.

(5) Based on a signal, i.e. timer position, etc., the controller 28energizes the valve 40 and deenergizes the valve 38; the gas on theopposite side of the piston 32 goes to atmosphere, allowing the gas fromthe chamber 18 to at once move the tip portion 22 to its open position.This allows gas to enter the sprue area 14 at once and flow into themolding.

(6) After the molding is filled out, the valve 26 is deenergized, thevalve 42 is energized and the gas is thereby vented to a lower moldingpressure. Alternately, the pressure of the gas is dropped as soon as thegas breaks through into the molding.

(7) After the molding is self-supporting, the valve 42 is deenergizedand the valve 48 is energized which will vent the molding to atmosphericpressure.

(8) After the molding has gone to atmospheric pressure, the valve 38 isenergized and the valve 40 is deenergized to close the valve mechanism24. The system is then ready for the next shot of plastic.

The above-noted procedure makes the gas ready close to the molding withlittle or no pressure drop as it flows into the molding and, therefore,there is no need to repressurize.

Referring now to FIGS. 5 through 7, there is illustrated a modified gasinjection bushing, generally indicated at 16', which may be locatedadjacent the mold cavity or the runner system in the mold or article(i.e. molding). Parts which have the same or similar function to theparts previously described have a prime designation.

The passageway 30' is closed at one location by a plug or weld 50.

A plate 52 of the modified bushing 16' is secured to the rest of the gasinjection bushing 16' by a bolt 54.

Gas fittings 56 and gas pipes 58 are provided to communicate gas topassages 30' and 34' as described with reference to the firstembodiment.

A cover plate 60 secures the plate 52 to the mold 61 by means of bolts62.

The invention has been described in an illustrative manner and, it is tobe understood that, the terminology which has been used is intended tobe in the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method for injection molding of plasticarticles in an injection molding system including a mold having aninjection aperture and a mold cavity for receiving molten resin, themethod comprising the steps of:(a) injecting a quantity of moltenplastic through the injection aperture and into the mold cavity, a resinflow path fluidly communicating the injection aperture and the moldcavity within the mold; (b) communicating a charge of pressurized fluidfrom the source of pressurized fluid to a chamber within the mold, anorifice opening from the chamber to the resin flow path; (c) blockingthe orifice so that the charge of pressurized fluid is stored within thechamber; (d) unblocking the orifice so that the stored charge ofpressurized fluid is introduced into the resin flow path from theorifice; (e) containing the fluid under pressure within the mold untilthe article is set up and formed in the mold cavity; and (f) venting thefluid from the mold wherein the mold includes valve means and whereinthe step of blocking is accomplished by applying a control pressure tothe valve means to block the orifice and the stored charge ofpressurized fluid causing the valve means to unblock the orifice uponrelieving the control pressure.
 2. The method of claim 1 wherein thevalve means includes a piston and a tip portion operatively associatedwith the piston to move therewith within the chamber, the tip portionhaving a closed position and an open position to introduce the charge ofpressurized fluid from the chamber, through the orifice and into theresin flow path, and wherein the step of blocking is accomplished byapplying the control pressure to the piston to cause the tip portion tomove to its closed position, the piston under pressure from the storedcharge of pressurized fluid causing the tip portion to move to its openposition to unblock the orifice upon relieving the control pressure. 3.The mothod of claim 2 wherein the tip portion extends through theorifice and into the flow path in its closed position.
 4. The method ofclaim 2 wherein the piston is mounted for reciprocating movement withinthe chamber.
 5. The method of claims 1 or 4 wherein the fluid is ventedfrom the article through the same orifice as which it was introduced.